A solid polymer electrolyte fuel cell has been expected to be used in wide fields such as a domestic co-generation power source, a power source for mobile instruments, a power source for electric automobiles and a simple auxiliary power source, because of its high energy density. In the solid polymer electrolyte fuel cell, an electrolyte membrane functions as an electrolyte for conducting protons, and simultaneously plays a role of a diaphragm for preventing hydrogen or methanol which is a fuel from being directly mixed with oxygen. Such an electrolyte membrane is required to have high ion-exchange capacity as an electrolyte, to have high proton conductivity, to be electrochemically stable and low in electric resistance because of passing an electric current for a long period of time, to have high mechanical strength as a membrane, and to have low gas permeability to hydrogen gas or methanol which is a fuel and to oxygen gas.
A perfluorosulfonic acid membrane (NAFION, a registered trade mark of DuPont) developed by DuPont, and the like have been generally used as such an electrolyte membrane for a fuel cell. However, in conventional fluorine-containing polymer ion-exchange membranes including NAFION, there have been observed problems that ion-exchange capacity is low, whereas chemical stability is excellent, and further that drying of the ion-exchange membranes occurs, resulting in a decrease in proton conductivity, because of insufficient water retention thereof. When a lot of sulfonic acid groups are introduced into the membrane as a countermeasure thereagainst, membrane strength remarkably decreases due to water retention, resulting in easy breakage of the membrane, which causes a problem of difficult compatibility of proton conductivity and membrane strength. Further, when a lot of sulfonic acid groups are introduced, the rate of change in membrane area also increases with an increase in water content of the membrane. In that case, for example, a direct methanol fuel cell (DMFC) in which an aqueous methanol solution is used as fuel has a problem that an electrolyte membrane which has come into contact with the fuel swells to cause a decrease in adhesion with an electrode. Further, the fluorine-containing polymer electrolyte membrane such as NAFION is very expensive because of complicated synthesis of a fluorine monomer used as a raw material, so that this is a great obstacle to practical realization of the solid polymer fuel cell.
For that reason, development of low-cost high-performance polymer electrolyte membranes has been advanced, in place of the fluorine-containing polymer electrolyte membranes including NAFION. For example, there have been proposed a polymer electrolyte membrane and the like synthesized by introducing a styrene monomer into an ethylene tetrafluoroethylene copolymer (ETFE) membrane by radiation graft reaction, followed by sulfonation (for example, patent document 1).
However, even the above-mentioned electrolyte membrane is not possible to solve the problem of a decrease in adhesion with the electrode.    Patent Document 1: JP-A-9-102322